This invention relates to a low-velocity electron excited phosphor of a blue luminous color, and more particularly to a low-velocity electron excited phosphor capable of exhibiting a blue luminous color at an anode voltage as low as 100 V or less in a fluorescent display device, a fluorescent luminous device, a write head for a printer, a back light for liquid crystal or the like.
The assignee proposed a phosphor of a blue luminous color which is constructed in such a manner that a matrix represented by a formula ZnO.GaO.sub.2 is doped with Li and P, as disclosed in Japanese Patent Application No. 331358/1987.
In the phosphor proposed, the matrix is formed by mixing ZnO and Ga.sub.2 O.sub.3 with each other in equivalent moles and then subjecting the resultant mixture to burning in an air atmosphere. Then, Li.sub.3 PO.sub.4 is added to the matrix thus obtained, followed by secondary burning of the matrix in a reducing atmosphere, to thereby permit the matrix to be doped with Li and P, resulting in preparing the phosphor.
The above-described procedure wherein ZnO and Ga.sub.2 O.sub.3 are mixed together in equivalent moles and then the resultant mixture is burned in an air atmosphere would lead to formation of a desired ZnO.Ga.sub.2 O.sub.3 matrix. Unfortunately, the burning in an air atmosphere causes Zn in ZnO to be evaporated due to decomposition, to thereby substantially fail to provide the matrix of a constant composition.
Also, the secondary burning in a reducing atmosphere after addition of Li.sub.3 PO.sub.4 in an amount as much as 5.times.10.sup.-3 to 4.times.10.sup.-1 mol per mol of the matrix causes the phosphor synthesized to exhibit only a tone of blue inclining to purple.
The inventors made an analysis of the conventional phosphor described above in order to solve such problems. As a result, it was found that the quantity of Li acting as an activator which is contained in the phosphor is as low as 500 ppm or less. The reason would be that ZnO was charged in a relatively large amount for synthesis of the matrix because of being easy to be evaporated. Charging of ZnO in a large amount causes ZnO which is not changed to a solid solution to remain even during the secondary burning. Thus, substitution between Zn and Li takes place under the conditions that Zn is present in an amount sufficient to render the replacement difficult. This results in the phosphor prepared being deteriorated in luminance and exhibiting a tone of blue inclining to purple due to an increase in component of a short wavelength of the tone.